...different moves have different costs. When you drop to zero Initiative, you still have access to your "spam skill" or the 1-slot attack skill which doesn't cost any resources.

You'll then have to wait a few seconds to regenerate enough Initiative to use a move that costs something.

Essentially, Thieves only ever have to wait a maximum of 5 seconds before being able to use any Weapon Skill in their arsenal.

But there are plenty of things to do while waiting for Initiative to regenerate; 1-skill spam, dodge rolling, using Steal offensively, using Utilities and Elite skills, etc. Running out of Initiative won't be anything more than a minor hindrance on you.